A shackle, commonly also referred to as a clevis or shank, is composed, as known, of a half-ring usually in steel with a U or horseshoe shape, wherein the ends of the legs are widened and perforated to allow the passage of a bolt for closure which can be threaded to screw into one of the holes, also threaded, or which alternatively can be locked to the relative leg by means of a split pin. In nautical applications a shackle is widely used for example for connecting two chain sections one to the other, for securing a chain to a fixed eye or to the anchor, or to a buoy.
As is known, a shackle is attached by linking the bolt to the chain link to be hooked, and this, for standard chains wherein the span in the link theoretically extends axially for a section equal to three times the diameter of the link, means in practice that the diameter of the bolt of the shackle has to be smaller than that of the chain link.
The span actually available for hooking the bolt, having subtracted the overall dimension of the arch of the links chained to the link to be hooked, is slightly smaller than the diameter of the link, due both to the curve of the arches of the links which are chained and to the thickness of additional zinc-plating of these same links. This leads to the need to use a smaller shackle. Naturally the presence of a smaller shackle causes a weakening of the hooking to the link of the chain, and the relevant bolt becomes the most fragile point of breakage from tensile stress.